U.S. patent application number 14/422892 was filed with the patent office on 2015-08-06 for machine part and process for producing same.
The applicant listed for this patent is NTN CORPORATION. Invention is credited to Takehiro Matsuduki, Toshihiko Mouri, Shinichi Nanbu, Fuminori Satoji, Makoto Shiranami.
Application Number | 20150217372 14/422892 |
Document ID | / |
Family ID | 50149792 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150217372 |
Kind Code |
A1 |
Mouri; Toshihiko ; et
al. |
August 6, 2015 |
MACHINE PART AND PROCESS FOR PRODUCING SAME
Abstract
Provided is a gear (1) (machine part), including a sintered
compact obtained by sintering a green compact of raw material
powder (10) that contains as a main raw material iron-based alloy
powder containing molybdenum and has 0.1 to 0.8 mass % of carbon
powder blended therein, the gear (1) (machine part) including a
hardened layer (8) formed through heat treatment after the
sintering, and having a true density ratio of 97% or more and less
than 100%.
Inventors: |
Mouri; Toshihiko; (Aichi,
JP) ; Matsuduki; Takehiro; (Shizuoka, JP) ;
Nanbu; Shinichi; (Aichi, JP) ; Shiranami; Makoto;
(Aichi, JP) ; Satoji; Fuminori; (Aichi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NTN CORPORATION |
Osaka |
|
JP |
|
|
Family ID: |
50149792 |
Appl. No.: |
14/422892 |
Filed: |
July 23, 2013 |
PCT Filed: |
July 23, 2013 |
PCT NO: |
PCT/JP2013/069857 |
371 Date: |
February 20, 2015 |
Current U.S.
Class: |
428/546 ; 419/11;
75/243 |
Current CPC
Class: |
B22F 7/00 20130101; B22F
2998/10 20130101; B22F 3/12 20130101; B22F 3/24 20130101; F16H
55/06 20130101; C22C 38/00 20130101; C22C 38/08 20130101; C22C
38/22 20130101; B22F 2998/10 20130101; B22F 5/08 20130101; B22F
3/162 20130101; C22C 33/02 20130101; C22C 38/12 20130101; B22F 3/10
20130101; B22F 3/02 20130101; Y10T 428/12014 20150115; B22F
2003/248 20130101; B22F 2003/248 20130101; B22F 1/0003
20130101 |
International
Class: |
B22F 7/00 20060101
B22F007/00; B22F 3/24 20060101 B22F003/24; C22C 33/02 20060101
C22C033/02; B22F 5/08 20060101 B22F005/08; C22C 38/12 20060101
C22C038/12; C22C 38/00 20060101 C22C038/00; B22F 3/12 20060101
B22F003/12; B22F 1/00 20060101 B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 23, 2012 |
JP |
2012-184184 |
Claims
1. A machine part, comprising a sintered compact obtained by
sintering a green compact of raw material powder that contains as a
main raw material iron-based alloy powder containing molybdenum and
has 0.1 to 0.8 mass % of carbon powder blended therein, the machine
part including a hardened layer formed through heat treatment after
the sintering, and having a true density ratio of 97% or more and
less than 100%.
2. The machine part according to claim 1, wherein at least a part
of the hardened layer comprises a dense layer in which a porous
structure is more densified than in another area.
3. The machine part according to claim 1, wherein a hardness of the
machine part gradually reduces from a surface to a core
portion.
4. The machine part according to claim 1, wherein the iron-based
alloy powder contains 0.2 to 0.8 mass % of molybdenum, given that a
mass of the raw material powder is defined as 100.
5. The machine part according to claim 1, wherein the iron-based
alloy powder further contains 0.2 to 0.8 mass % of nickel, given
that a mass of the raw material powder is defined as 100.
6. A method of manufacturing a machine part comprising a sintered
compact having a true density ratio of 97% or more and less than
100%, the method comprising: a compression molding step of
compression molding raw material powder that contains as a main raw
material iron-based alloy powder containing molybdenum and has 0.1
to 0.8 mass % of carbon powder blended therein, in a mold to obtain
a green compact; a sintering step of heating the green compact at a
temperature equal to or higher than a sintering temperature of the
iron-based alloy powder to obtain a sintered compact; and a heat
treatment step of subjecting the sintered compact to heat treatment
to form a hardened layer.
7. The method of manufacturing a machine part according to claim 6,
wherein the compression molding step comprises compression molding
the raw material powder in a state in which a solid lubricant
adheres to a molding surface of the mold.
8. The method of manufacturing a machine part according to claim 6,
wherein the raw material powder to be used has a solid lubricant
added thereto.
9. The method of manufacturing a machine part according to claim 7,
wherein the compression molding of the raw material powder is
performed in a state in which the mold is heated.
10. The method of manufacturing a machine part according to claim
6, further comprising a plastic processing step of subjecting the
sintered compact to plastic processing between the sintering step
and the heat treatment step.
11. The method of manufacturing a machine part according to claim
10, wherein the plastic processing comprises rolling
processing.
12. The method of manufacturing a machine part according to claim
8, wherein the compression molding of the raw material powder is
performed in a state in which the mold is heated.
Description
TECHNICAL FIELD
[0001] The present invention relates to a machine part and a
manufacturing method therefor, and more particularly, to a machine
part formed of a sintered metal and a manufacturing method
therefor.
BACKGROUND ART
[0002] For example, a machine part to be used for a power
transmission application (power transmission part) or a machine
part to be used in a part subjected to pressure in a pump or the
like (pressure receiving part) is required to have particularly
high mechanical strength, abrasion resistance, fatigue strength,
and the like. Therefore, a machine part of this kind is generally
formed of an ingot material. However, in order to obtain a machine
part formed of an ingot material satisfying the above-mentioned
demand characteristics with high accuracy, a number of processing
steps are required. Further, a great material loss is caused in
such steps. Therefore, a machine part obtained by processing an
ingot material is generally liable to have high cost. In such
circumstances, various attempts have recently been made to produce
the machine part as described above by using a sintered metal
(sintered material).
[0003] For example, in Patent Literature 1, the applicant of the
present application has proposed a machine part (power transmission
part), which is formed of a sintered material obtained by molding
granulated powder of fine powders each containing iron as a main
component into a green compact, followed by sintering, and which is
obtained by subjecting the sintered material to heat treatment
(quenching and tempering treatment). The sintered material to be
used as a base material for the machine part in Patent Literature 1
has a high density (a true density ratio of 85% or more) as
compared to a general sintered material because of, for example,
the following reasons: (1) the sintered material is produced by
using as a main raw material the granulated powder obtained through
granulation of the fine powders so as to have an appropriate grain
size, and hence exhibits improved flowability in a mold, and by
extension, improved moldability even though the fine powders are
used; and (2) the granulated powder obtained through granulation of
the fine powders has a large surface area, and hence sintering
property between adjacent granulated powders is improved.
Therefore, the machine part formed of such sintered material has
improved mechanical strength, abrasion resistance, and the like as
compared to a machine part formed of the general sintered material.
In addition, the sintered material proposed in Patent Literature 1
can be manufactured through similar steps to those in the case of
the general sintered material except that raw material powder
containing as a main raw material the granulated powder is used. In
view of the foregoing, when a construction of Patent Literature 1
is adopted, it is considered that a machine part having improved
demand characteristics including mechanical strength and abrasion
resistance can be produced with high accuracy at relatively low
cost.
CITATION LIST
[0004] Patent Literature 1: JP 2011-94789 A
SUMMARY OF INVENTION
Technical Problem
[0005] However, the machine part formed of a sintered material
proposed in Patent Literature 1 is still far inferior in mechanical
strength, abrasion resistance, and the like to a machine part
formed of an ingot material owing to its true density ratio being
set to 85% or more. Therefore, its application is inevitably
limited. In addition, in the construction of Patent Literature 1,
it is essential to produce (prepare) the granulated powder of high
quality in order to enable stable manufacturing (mass-production)
of a desired machine part, but a lot of labor and cost are required
for stably obtaining such granulated powder of high quality.
Therefore, the construction of Patent Literature 1 cannot achieve a
cost reduction effect as much as expected, and the cost of the
machine part may be increased instead.
[0006] In view of such circumstances, an object of the present
invention is to provide a machine part formed of a sintered metal,
which can be mass-produced at low cost and is excellent in
mechanical strength, abrasion resistance, and the like.
Solution to Problem
[0007] According to one embodiment of the present invention, which
has been made to achieve the above-mentioned object, there is
provided a machine part, comprising a sintered compact obtained by
sintering a green compact of raw material powder that contains as a
main raw material iron-based alloy powder containing molybdenum and
has 0.1 to 0.8 mass % of carbon powder blended therein, the machine
part including at least a hardened layer formed through heat
treatment, and having a true density ratio of 97% or more and less
than 100%.
[0008] The true density ratio as used herein is represented by the
calculation equation described below.
True density ratio=(density of entire machine part formed of
sintered compact/true density).times.100 [%]
[0009] It should be noted that the "true density" in the equation
means a theoretical density of a material having no pores in its
interior portion, such as an ingot material. In addition, the
"density of entire machine part formed of sintered compact" in the
equation is measured by, for example, a method specified in JIS
Z2501.
[0010] In addition, according to one embodiment of the present
invention, which has been made to achieve the above-mentioned
object, there is provided a method of manufacturing a machine part
comprising a sintered compact having a true density ratio of 97% or
more and less than 100%, the method comprising: a compression
molding step of compression molding raw material powder that
contains as a main raw material iron-based alloy powder containing
molybdenum and has 0.1 to 0.8 mass % of carbon powder blended
therein, in a mold to obtain a green compact; a sintering step of
heating the green compact at a temperature equal to or higher than
a sintering temperature of the iron-based alloy powder to obtain a
sintered compact; and a heat treatment step of subjecting the
sintered compact to heat treatment to form a hardened layer.
[0011] As described above, the machine part formed of a sintered
metal according to one embodiment of the present invention has a
true density ratio of 97% or more and less than 100% (roughly
corresponding to 7.6 g/cm.sup.3 or more and less than 7.8
g/cm.sup.3, when converted into a density), and is highly densified
to the extent approximate to that in the case of an ingot material.
Therefore, the machine part exhibits excellent characteristics in
mechanical strength, abrasion resistance, fatigue strength, and the
like.
[0012] As a result of diligent studies, the inventors of the
invention of the present application have found that such machine
part formed of a sintered metal having a high density and high
strength can be obtained by adopting using the sintered compact
(sintered material) including a hardened layer formed through heat
treatment after the sintering, and further adopting the following
constructions (1) and (2) in a material aspect: (1) selectively
using as main powder constituting the raw material powder the
iron-based alloy powder containing molybdenum; and (2) setting the
blending ratio of the carbon powder in the raw material powder to
0.1 to 0.8 mass %, which is lower than that in a general sintered
metal. Specifically, the adoption of the above-mentioned
construction (1) improves compression moldability and sintering
property. The adoption of the above-mentioned construction (2)
further improves the compression moldability along with a reduction
in the blending amount of carbon having a low specific gravity,
while securing lubricity between powders and heat treatment
property (heat treatment property after the sintering). Thus, a
high density is achieved.
[0013] On the other hand, the use of the raw material powder
containing the iron-based alloy powder as a main raw material and
having an appropriate amount of carbon powder blended therein, as
the raw material powder for obtaining the machine part according to
one embodiment of the present invention eliminates the need for
troublesome treatment such as granulation treatment proposed in
Patent Literature 1 in the process of preparing and producing the
raw material powder. Therefore, the manufacturing cost can be
significantly reduced as compared to the construction of Patent
Literature 1. As described above, according to one embodiment of
the present invention, it is possible to obtain the machine part
formed of a sintered metal, which can be mass-produced at low cost
and is excellent in mechanical strength, abrasion resistance, and
the like.
[0014] Herein, a method for the above-mentioned heat treatment is
not particularly limited, and any known quenching method such as
carburizing quenching, immersion quenching, or high-frequency
quenching can be appropriately selected. It should be noted that
tempering is preferably performed in combination after the
quenching in order to provide the machine part with high toughness
as well.
[0015] In the above-mentioned manufacturing method according to one
embodiment of the present invention, the compression molding step
may comprise compression molding the raw material powder in a state
in which a solid lubricant adheres to a molding surface of the
mold. Further, in addition to or instead of the foregoing, the raw
material powder to be used may have a solid lubricant added
thereto. This improves the lubricity between the raw material
powder and the mold or the lubricity between the powders, and hence
provides an advantage in obtaining the green compact having a high
density, and by extension, the machine part formed of a sintered
metal having high strength. It should be noted that, when the raw
material powder to be used has a solid lubricant added thereto, the
addition amount of the solid lubricant is preferably as low as
possible from the viewpoint of highly densifying the green compact.
Specifically, the addition amount of the solid lubricant is
preferably 0.1 mass % or less, more preferably 0.05 mass % or less
in terms of outer percentage, given that the mass of the raw
material powder is defined as 100%.
[0016] When the compression molding of the raw material powder is
performed in a state in which a solid lubricant adheres to a
molding surface of the mold and/or the raw material powder to be
used has a solid lubricant added thereto, the compression molding
of the raw material powder is preferably performed in a state in
which the mold is heated (preferably heated to a temperature equal
to or higher than the melting point of the solid lubricant). This
allows a lubricant component to diffuse and penetrate with high
efficiency. Therefore, the addition amount (blending amount) of the
solid lubricant can be reduced. Thus, the green compact having a
high density, and by extension, the machine part having high
strength can be easily obtained at low cost.
[0017] In the machine part having the above-mentioned construction,
at least a part of the hardened layer may comprise a dense layer in
which a porous structure is more densified than in another area.
With this, the machine part can achieve a higher density and higher
strength.
[0018] It should be noted that such machine part may be obtained by
performing a plastic processing step of subjecting the sintered
compact to plastic processing between the sintering step and the
heat treatment step. As the plastic processing in the plastic
processing step, for example, rolling can be adopted. However, the
processing method to be adopted is appropriately selected depending
on, for example, the shape of the machine part. Whatever processing
method is adopted, cold plastic processing can efficiently improve
the accuracy and density (strength) of a portion to be processed as
compared to warm or hot plastic processing.
[0019] The hardness of the machine part having the above-mentioned
construction may gradually reduce from a surface to a core portion
(a center portion in the thickness direction). This can provide the
machine part with toughness, and hence prolong the durability
lifetime of the machine part.
[0020] In the machine part having the above-mentioned construction,
as the iron-based alloy powder constituting the raw material
powder, there may be used iron-based alloy powder containing 0.2 to
0.8 mass % of molybdenum, given that the mass of the raw material
powder is defined as 100. In addition, as the iron-based alloy
powder, there may be used iron-based alloy powder further
containing 0.2 to 0.8 mass % of nickel (Ni), given that the mass of
the raw material powder is defined as 100. In addition, the raw
material powder may further contain chromium (Cr) powder, manganese
sulfide (MnS) powder, or the like.
[0021] The present invention described above can be preferably
applied to the production of a machine part such as a gear or a cam
by using a sintered metal. Needless to say, the present invention
is not only applied to a gear, a cam, or the like and can also be
preferably applied to the production of any other machine part (for
example, a bearing) by using a sintered metal.
Advantageous Effects of Invention
[0022] As described above, according to one embodiment of the
present invention, it is possible to mass-produce the machine part
formed of a sintered metal exhibiting excellent characteristics in
mechanical strength, abrasion resistance, and the like at low
cost.
BRIEF DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a perspective view of a gear as a machine part
according to one embodiment of the present invention.
[0024] FIG. 2 is an enlarged sectional view of a portion X
illustrated in FIG. 1.
[0025] FIG. 3 is a block diagram illustrating a manufacturing
procedure of the gear illustrated in FIG. 1.
[0026] FIG. 4 is a schematic sectional view illustrating an initial
stage of a compression molding step.
[0027] FIG. 5 is a schematic sectional view illustrating a middle
stage of the compression molding step.
[0028] FIG. 6 is an enlarged view of a main portion illustrating
one embodiment of a plastic processing step.
[0029] FIG. 7 is a view schematically illustrating a state in which
the shape of a sintered compact changes along with plastic
processing.
[0030] FIG. 8 is a graph showing actually measured results of the
hardness of the sintered compact before and after the plastic
processing.
DESCRIPTION OF EMBODIMENTS
[0031] Embodiments of the present invention are hereinafter
described with reference to the drawings.
[0032] FIG. 1 is a perspective view of a machine part according to
one embodiment of the present invention, and FIG. 2 is an enlarged
sectional view (enlarged sectional view in the plane surface
perpendicular to the axis) of a portion X illustrated in FIG. 1.
The machine part illustrated in FIG. 1 is, for example, a gear
(gear for a transmission) 1 to be used by being incorporated into a
transmission of an automobile. The gear 1 includes a large-diameter
annular portion 3 having a tooth surface 2 along its outer
circumference, a small-diameter annular portion 4 having a mounting
hole for mounting the gear 1 to a rotation shaft (not shown), and a
plurality of connecting portions 5 for connecting the
large-diameter annular portion 3 and the small-diameter annular
portion 4, in an integrated manner.
[0033] The gear 1 is formed of a sintered metal and has a true
density ratio of 97% or more and less than 100%. A hardened layer
(surface hardened layer) 8 is formed in the entire area of the gear
1 through heat treatment after sintering (by subjecting a sintered
compact to heat treatment). In addition, as illustrated in FIG. 2,
a dense layer 7 in which a porous structure is more densified than
in another area is formed in the area in which the tooth surface 2
is formed (outer circumference of the large-diameter annular
portion 3) in the gear 1. The dense layer 7 is formed by subjecting
the outer circumference of a sintered compact to plastic
processing, and the hardened layer 8 is formed by subjecting the
sintered compact after the plastic processing (after the formation
of the dense layer 7) to heat treatment, while the methods of
forming the dense layer 7 and the hardened layer 8 are described in
detail later. That is, the hardened layer 8 is formed so as to
include the dense layer 7. For such construction, the dense layer 7
has a higher hardness than the hardened layer 8. In addition, in
each of the portions 3 to 5 of the gear 1, the hardness is the
highest in the surface and gradually reduces toward the core
portion (center portion in the thickness direction). Therefore, the
tooth surface 2 has the highest mechanical strength, the highest
abrasion resistance, and the like of the portions constituting the
gear 1.
[0034] The gear 1 as the machine part having the above-mentioned
construction is manufactured by a raw material powder preparation
step S1, a compression molding step S2, a sintering step S3, a
plastic processing step S4, and a heat treatment step S5 in the
stated order, as illustrated in FIG. 3. The steps are hereinafter
described in that order.
[0035] In the raw material powder preparation step S1, raw material
powder 10 (see FIG. 5) as a material for forming the gear 1 is
prepared and produced. Herein, as the raw material powder 10, there
is used raw material powder that contains as a main raw material
(main powder) iron-based alloy powder containing molybdenum (Mo)
and nickel (Ni) and has an appropriate amount of carbon (C) powder
blended therein. As the iron-based alloy powder described above,
partially alloyed powder or completely alloyed powder of
molybdenum, nickel, and iron (Fe) is used. The ratios of the
elements in the raw material powder 10 are as follows: 0.2 to 0.8
mass % of molybdenum, 0.2 to 0.8 mass % of nickel, and 0.1 to 0.8
mass % of carbon, with the balance being iron (and inevitable
impurities), given that the mass of the raw material powder 10 is
defined as 100. Herein, raw material powder in which the blending
ratios of molybdenum, nickel, and carbon are each 0.5 mass %, with
the balance being iron (and inevitable impurities) is used.
[0036] As the iron-based alloy powder, one having an average grain
size of from 40 to 150 .mu.m is used in consideration of cost and
compression moldability. Specifically, small-size iron-based alloy
powder having an average grain size of less than 40 .mu.m is
expensive, and reduces the compression moldability owing to its
poor flowability. In contrast, when large-size iron-based alloy
powder having an average grain size exceeding 150 .mu.m is used, it
is difficult to obtain a green compact having a high density
because large pores are liable to be formed between the
powders.
[0037] One kind or two or more kinds of solid lubricants, such as
zinc stearate and molybdenum disulfide, may be added (blended in
terms of outer percentage) to the raw material powder 10. In this
embodiment, less than 0.05 mass % of zinc stearate is added to the
raw material powder 10, given that the mass of the raw material
powder 10 is defined as 100.
[0038] Next, in the compression molding step S2, the raw material
powder 10 is subjected to compression molding by using a molding
device 11 as illustrated in FIGS. 4 and 5, to provide a green
compact having a shape close to that of the gear 1 illustrated in
FIG. 1 (substantially finished product shape). The molding device
11 includes, as main constituents, a mold 12 having a core 13,
upper and lower punches 14, 15, and a die 16, which are disposed
coaxially, a heater 17 for heating the die 15 (mold 12), and a
lubricant injection nozzle 18 for injecting a solid lubricant 19
into a cavity 12a of the mold 12. The mold 12 is set in, for
example, a CNC press machine using as a drive source a servomotor
(not shown). In addition, the mold 12 may be equipped with a
vibration imparting device for imparting vibration in order to
enhance filling property of the raw material powder 10, while the
illustration is omitted.
[0039] In the molding device 11 having the above-mentioned
construction, first, the solid lubricant 19 is injected from the
lubricant injection nozzle 18 arranged between the core 13 and the
upper punch 14 into the cavity 12a as illustrated in FIG. 4. Thus,
the solid lubricant 19 is allowed to adhere to the surfaces
defining the cavity 12a (a radially outer surface 13a of the core
13, an upper edge surface 15a of the lower punch 15, and a radially
inner surface 16a of the die 16) (see the enlarged view in FIG. 5).
As the solid lubricant 19, there may be used any lubricant of the
same kind as or different kind from the solid lubricant added to
the raw material powder 10. In addition, there may be used not only
one kind, but a mixture of two or more kinds Herein, a solid
lubricant of the same kind as the solid lubricant added to the raw
material powder 10, that is, zinc stearate is used as the solid
lubricant 19.
[0040] After the solid lubricant 19 is allowed to adhere to the
surfaces defining the cavity 12a as described above, the lubricant
injection nozzle 18 is moved backward and the raw material powder
10 is loaded and filled into the cavity 12a. Next, the upper punch
14 is relatively moved so as to be close to the lower punch 15, to
pressurize the raw material powder 10 filled into the cavity 12 at
a pressure of, for example, from 800 to 1,100 MPa. Thus, the green
compact is formed. Then, the upper punch 14 is moved upward, and
concurrently the lower punch 15 is moved upward, to discharge the
green compact out of the cavity 12a (to release the green compact
from the mold).
[0041] Herein, in the process for forming the green compact
described above, the heater 17 is activated to heat the die 16
(mold 12) at least a time period from the filling of the raw
material powder 10 into the cavity 12a to the completion of the
formation of the green compact. Specifically, the green compact is
obtained by subjecting the raw material powder 10 filled into the
cavity 12a to compression molding in the state in which the mold 12
is heated. The heating of the die 16 by the heater 17 is performed
so that the temperature of the die 16 is 70.degree. C. or more,
more preferably a temperature equal to or higher than the melting
point of the solid lubricant 19 and 120.degree. C. or less. With
this, a lubricant component of the solid lubricant 19 adhering to
the surfaces defining the cavity 12a can diffuse and penetrate
efficiently between the raw material powder 10 (green compact) and
the cavity 12a, further between the powders constituting the raw
material powder 10. Therefore, the moldability and mold
releasability of the green compact are improved even when the
amount of the solid lubricant 19 to adhere to the surfaces defining
the die 12a or the amount of the solid lubricant to be added to the
raw material powder 10 is reduced. Accordingly, this provides an
advantage in obtaining the green compact having a high density and
high accuracy, and by extension, the gear 1 having high
strength.
[0042] It should be noted that the heating of the die 16 by the
heater 17 may be performed so as to keep the die 16 in the
above-mentioned temperature range at all times. In addition, while
this embodiment adopts a so-called internal heating method of
heating the die 16 (mold 12) by the heater 17 disposed inside the
die 16, an external heating method of heating the mold 12 by the
heater or the like disposed outside the mold 12 may be adopted. In
addition, the raw material powder 10 preliminarily heated may be
filled into the cavity 12a of the mold 12.
[0043] The green compact thus obtained is transferred to the
sintering step S3. In the sintering step S3, the green compact is
heated at a temperature equal to or higher than the sintering
temperature of the iron-based alloy powder constituting the green
compact, to bind by sintering the powders adjacent to each other.
Thus, a sintered compact 1' is obtained (see FIG. 6). Sintering in
an active gas atmosphere such as oxygen may cause oxidation of the
powder constituting the green compact and adversely affect the
mechanical strength and the like of the sintered compact 1', and by
extension, of the gear 1, because the green compact is obtained
through compression molding of the raw material powder 10
containing as a main raw material the iron-based alloy powder.
Therefore, in the sintering step S3, the green compact is placed in
an inert gas atmosphere (for example, nitrogen gas atmosphere) and
heated at 1,200.degree. C. or more and 1,300.degree. C. or less
(for example, 1,250.degree. C.) for a predetermined time period, to
provide the sintered compact 1'. The reason why the lower limit of
the heating temperature is set to 1,200.degree. C. is that, when
the green compact is heated at a lower temperature (for example,
1,120.degree. C., which is a temperature for forming a sintered
compact of a general iron-based metal), the powders fail to be
bound to each other with sufficient binding strength. In addition,
the reason why the upper limit of the heating temperature is set to
1,300.degree. C. is that a strength improving effect is saturated.
It should be noted that the green compact may be sintered in
vacuum, not in the inert gas atmosphere as described above.
[0044] The sintered compact 1' thus obtained is transferred to the
plastic processing step S4. In the plastic processing step S4, the
sintered compact 1' is subjected to plastic processing to form the
tooth surface 2 in a finished shape on the outer circumference of
the sintered compact 1'. The formation of the tooth surface 2 is
performed by, for example, using a rolling machine 20 as
schematically illustrated in FIG. 6. The rolling machine 20
includes a support shaft 23 for supporting the sintered compact 1'
in a rotatable manner, two roller dies 21, 22 each arranged at the
radially outer side of the support shaft 23 and having a forming
portion for forming the tooth surface 2 on its outer circumference.
The two roller dies 21, 22 are arranged so as to face each other
across the support shaft 23 (sintered compact 1'). In such rolling
machine 20, the roller dies 21, 22 are rotated while being pressed
against the outer circumference of the sintered compact 1'
supported by the support shaft 23 in a rotatable manner (rotated in
a counterclockwise direction in the example illustrated in FIG. 6).
With this, the tooth surface 2 in a finished shape is formed on the
entire circumference of the the sintered compact 1'. It should be
noted that the rolling processing described above is performed on,
for example, the sintered compact 1' including a tooth surface 2'
constructed by consecutively providing tooth portions each having
an involute shape (see the two-dot chain line in FIG. 7). In this
case, the tooth portions constituting the tooth surface 2' are each
subjected to compression deformation in the circumferential
direction and elongation deformation in the radial direction along
with the rolling processing, to be formed into a tooth portion in a
finished shape illustrated by the solid line in FIG. 7.
[0045] In addition, along with this, the porous structure on the
outer circumference of the sintered compact 1' is densified to form
the dense layer 7 (see FIG. 2). When the porous structure is
densified to form the dense layer 7 as described above, pores in
which stress is to be concentrated are reduced, and thus, the tooth
surface 2 excellent in mechanical strength, particularly in fatigue
strength, is obtained.
[0046] The rolling processing (plastic processing step S4)
described above may be performed in any temperature range for cold
processing, warm processing, or hot processing. It should be noted
that cold rolling processing is preferred from the viewpoints of
improving the formation accuracy of the tooth surface 2 and
prolonging the durability lifetime of the roller dies 21, 22.
[0047] Now, FIG. 8 shows measured results of the hardness of the
sintered compact 1' on its outer circumference before and after the
plastic processing. As apparent from FIG. 8, the hardness of the
sintered compact 1' on its outer circumference is significantly
increased after the rolling processing as compared to before the
rolling processing. The hardness significantly increases
particularly around the surface of the sintered compact 1',
gradually reduces therefrom to the core portion (inner portion) of
the sintered compact 1', and becomes almost constant beyond a
predetermined depth.
[0048] Through the steps described above, the sintered compact 1'
having a true density ratio of 97% or more and less than 100%,
herein having a true density ratio of 97% (roughly corresponding to
7.6 g/cm.sup.3 when converted into a density) is formed.
[0049] Finally, the sintered compact 1' having the tooth surface 2
in a finished shape formed therein is transferred to the heat
treatment step S5. In the heat treatment step S5, the sintered
compact 1' is subjected to heat treatment, to form the hardened
layer 8 on the entire surface area of the sintered compact 1'
associated with quenching (see FIG. 2). With this, the mechanical
strength and abrasion resistance of the sintered compact 1' are
entirely increased. In particular, the mechanical strength and
abrasion resistance of the area in which the dense layer 7 is
formed are further increased in the sintered compact 1'. The method
for the heat treatment is not particularly limited, and any known
method such as carburizing quenching, immersion quenching, or
high-frequency quenching can be appropriately adopted. Herein,
carburizing quenching is adopted. Tempering is performed after the
quenching in order to provide the sintered compact 1' (gear 1) with
high toughness as well as high mechanical strength and the
like.
[0050] Through the steps described above, the gear 1 as the machine
part as illustrated in FIGS. 1 and 2 is completed. After the heat
treatment step S5, with a view to further improving the accuracy of
the portions of the sintered compact 1' (gear 1), finishing
processing such as grinding processing, polishing processing,
lapping processing, or super finishing processing may be performed,
as required.
[0051] As described above, the gear 1 formed of a sintered metal
according to the present invention has a true density ratio of 97%
or more and less than 100% (roughly corresponding to 7.6 g/cm.sup.3
or more and less than 7.8 g/cm.sup.3 when converted into a density)
and is highly densified to the extent approximate to that in the
case of an ingot material. Therefore, the gear 1 exhibits excellent
characteristics in mechanical strength, abrasion resistance,
fatigue strength, and the like.
[0052] As a result of diligent studies, the inventors of the
invention of the present application have realized the sintered
compact 1' having a high density and high strength by adopting the
following constructions (1) and (2) particularly in a material
aspect: (1) selectively using as a main raw material constituting
the raw material powder 10 the iron-based alloy powder containing
molybdenum; and (2) setting the blending ratio of the carbon powder
in the raw material powder 10 to from 0.1 to 0.8 mass %, which is
lower than that in a general sintered metal. Specifically, the
adoption of the above-mentioned construction (1) improves the
compression moldability and the sintering property. The adoption of
the above-mentioned construction (2) further improves the
compression moldability along with a reduction in the blending
amount of carbon having a low specific gravity, while securing
lubricity between the powders and hardenability after sintering.
Thus, a high density is achieved.
[0053] In addition, the gear 1 according to the present invention
includes the hardened layer 8 formed through the heat treatment
after the sintering, and hence achieves high strength. Particularly
in this embodiment, the sintered compact 1' is subjected to
carburizing, quenching, and tempering as the heat treatment, and
hence the hardness of the gear 1 gradually reduces from the surface
to the core portion. Therefore, the gear 1 has high toughness as
well, and hence is excellent in durability lifetime and impact
resistance.
[0054] On the other hand, the use of the raw material powder
containing the iron-based alloy powder as a main raw material and
having an appropriate amount of the carbon powder blended therein,
as the raw material powder 10 for obtaining the gear 1 eliminates
the need for troublesome treatment such as granulation treatment
disclosed in Patent Literature 1 in the process of preparing and
producing the raw material powder 10. Therefore, manufacturing cost
can be significantly reduced as compared to the construction of
Patent Literature 1. As described above, according to the present
invention, it is possible to mass-produce the gear 1 formed of a
sintered metal that is excellent in mechanical strength, abrasion
resistance, fatigue strength, and the like and has high toughness
at low cost.
[0055] The gear 1 formed of a sintered metal and the manufacturing
method therefor according to embodiments of the present invention
have been described above, but the composition of the raw material
powder 10 suitable for manufacturing of the gear 1 is not limited
to the one described above.
[0056] For example, the raw material powder 10 that contains as a
main raw material iron based alloy powder containing molybdenum
(partially alloyed powder of molybdenum-iron) and has carbon powder
and chromium (Cr) powder blended therein may be used. In this case,
there may be given, for example, a construction in which the
powders are blended so that the contents of molybdenum, carbon, and
chromium may be from 0.2 to 0.8 mass %, from 0.1 to 0.8 mass %, and
from 0.5 to 2.0 mass %, respectively, and the balance may be iron
(and an unavoidable impurity), given that the mass of the raw
material powder 10 is defined as 100. In addition, for example, the
raw material powder 10 that contains as a main raw material iron
based alloy powder containing molybdenum and nickel (partially
alloyed powder of molybdenum-nickel-iron) and has carbon powder and
manganese sulfide (MnS) powder blended therein may also be used. In
this case, there may be given, for example, a construction in which
the powders are blended so that the contents of molybdenum, nickel,
carbon, and manganese sulfide may be from 0.2 to 0.8 mass %, from
0.2 to 0.8 mass %, from 0.1 to 0.8 mass %, and from 0.5 to 2.0 mass
%, respectively, with the balance being iron (and inevitable
impurities), given that the mass of the raw material powder 10 is
defined as 100.
[0057] In addition, the plastic processing step S4 may be omitted
while, in the embodiment described above, the plastic processing
step S4 is performed between the sintering step S3 and the heat
treatment step S5, and in the plastic processing step S4, the
sintered compact 1' is formed into a finished shape and the dense
layer 7 in which a porous structure is more densified than in
another area is formed. Specifically, the plastic processing step
S4 can be omitted when the green compact in a finished shape can be
formed through the compression molding step S2 and the sintered
compact 1' having a true density ratio of 97% or more and less than
100% can be obtained only by forming the hardened layer 8 through
the heat treatment step S5.
[0058] In addition, although the case in which the present
invention is applied to a gear for a transmission of an automobile
has been described above, the present invention can also be applied
to any other machine part such as a gear or a cam to be used by
being incorporated into various mechanical devices (for example, a
cam for a vane pump). In particular, the machine part according to
the present invention is formed of a sintered compact highly
densified to the extent approximate to that of an ingot material
and can be mass-produced at low cost while securing high mechanical
strength and high abrasion resistance, and hence is extremely
advantageous because the machine part can be applied to such
application in which replacement from a machine part formed of an
ingot material has hitherto been difficult and thus contribute to
reduction in cost of various mechanical devices.
REFERENCE SIGNS LIST
[0059] 1 gear (machine part) [0060] 1' sintered compact [0061] 2
tooth surface [0062] 7 dense layer [0063] 8 hardened layer [0064]
10 raw material powder [0065] 12 mold [0066] 12a cavity [0067] 17
heater [0068] 18 lubricant injection nozzle [0069] 19 solid
lubricant [0070] 20 rolling machine [0071] S1 raw material powder
preparation step [0072] S2 compression molding step [0073] S3
sintering step [0074] S4 plastic processing step [0075] S5 heat
treatment step
* * * * *